Monthly Archives: July 2014

The subject of this short posting is a public figure, Colin Powell, rather than a scientist. It deals with Powell’s strong support of condom usage by sexually active young people, so that they might protect themselves against contracting HIV/AIDS. In 2002, at the time of this particular incident, Powell was serving as Secretary of State in the George W. Bush administration. Importantly, Powell’s advocacy of condoms to prevent sexual transmission of HIV was contrary to the Bush administration’s strongly held abstinence-only approach. Powell, a Republican, previously served as a four-star general in the U.S. Army and as Chairman of the Joint Chiefs of Staff.

[Aside: This incident was brought to mind by the recent “Hobby Lobby” case, in which the conservative majority of the U.S. Supreme Court ruled by a 5-4 vote that certain so-called “closely held” corporations do not have to provide contraceptive coverage to their employees under the Affordable Care Act (“Obama care”), if providing that coverage violates the religious beliefs of the corporation’s owners.]

Powell made his remarks during an interview concerning the spread of AIDS that was broadcast on the MTV network. Among Powell’s comments, he said “It is important that the whole international community come together, speak candidly about it, forget taboos, and forget about conservative ideas about what you should tell young people about. It’s the lives of young people that are put a risk by unsafe sex.”

While the interview took place in a Washington D.C. studio, it was transmitted via satellites to studios worldwide, and Powell took questions from young listeners both at home and abroad. A young Italian Catholic woman asked Powell, through an interpreter, to specifically state his position regarding condoms. Powell replied “I certainly respect the position of the Holy Father and the Catholic Church. In my own judgment, condoms are a way to prevent infection. Therefore, I not only support their use, I encourage their use among people who are sexually active and need to protect themselves. I think it’s important for young people especially to protect themselves from the possibility of acquiring any sexually transmitted disease, but especially to protect themselves from HIV/AIDS, which is a plague that is upon the face of the earth.”

Not surprisingly, Powell’s statements generated an angry response from some of the President’s closest supporters, as well as from conservative politicians and right-wing religious groups. Most criticism took Powell to task on “moral” grounds. For example, the president of Concerned Women for America stated, “He undercut the moral authority of all parents, he embarrassed President Bush and undercut the Administration’s policies, and he needs to retract these statements immediately.”

Others of Powell’s detractors added that condom usage is not 100% effective at preventing sexual transmission of HIV. To that point, it is difficult to accurately measure the effectiveness of condoms, since that assessment involves looking into people’s private behaviors. Nevertheless, the U.S. Centers for Diseases Control and Prevention states: “The body of research on the effectiveness of latex condoms in preventing sexual transmission of HIV is both comprehensive and conclusive. The ability of latex condoms to prevent transmission of HIV has been scientifically established in ‘real-life’ studies of sexually active couples as well as in laboratory studies.” And, since condoms need to be used consistently and correctly to be maximally effective, it is important that sex education programs speak to these points. For, as Powell said, “It’s the lives of young people that are put at risk by unsafe sex.”

White House spokesman Ari Fleischer attempted to calm matters stating, “Colin Powell takes a back seat to no one when it comes to abstinence and abstinence education.”

In earlier postings, we recounted how in 1959 Bernice Eddy, at the U. S. National Institutes of Health (NIH), and then Maurice Hilleman, at Merck & Co, discovered a new virus, simian virus 40 (SV40), in early lots of the Salk and Sabin polio vaccines (1, 2). The virus inadvertently contaminated those vaccines because it was unknowingly present in the rhesus monkey kidney cell cultures in which the vaccines were grown. Hilleman gave SV40 its name. It was the 40th simian virus that the Merck lab found in its rhesus kidney cell cultures.

Next, in 1961, both Eddy and Hilleman discovered that inoculating SV40 into hamsters caused cancer in about half of the animals. But, by then, hundreds of millions of people worldwide had been inoculated with live SV40, via the contaminated polio vaccines!

[Aside 1: Since the Sabin vaccine contains live, attenuated poliovirus, whereas the Salk vaccine contained formaldehyde-inactivated poliovirus, it was initially thought that any issues stemming from SV40 contamination would be limited to the Sabin vaccine. However, about one in 10,000 SV40 particles survived the formaldehyde treatment used to inactivate poliovirus in the Salk vaccine. Thus, early lots of both the Salk vaccine and the Sabin vaccine were contaminated with live SV40. But, despite the initial belief that that only the Sabin vaccine posed a threat of SV40 contamination, recipients of the Salk vaccine, but not those receiving the Sabin vaccine, developed an antibody response against SV40. Apparently, any SV40 that may have been present in the orally administered Sabin vaccine was killed in the intestinal tracts of its recipients. But, while the antibody response seen in recipients of the injection-administered Salk vaccine would be consistent with an actual SV40 infection, that response might have been only against the SV40 in the vaccine itself (see the main text).]

In July 1961, the New York Times broke part of this story, reporting that Merck was withdrawing its vaccines because they were contaminated with a monkey virus. However, the Times article did not mention the cancer connection. In fact, the Times did not report that aspect of the story until more than a year later, causing some individuals to suspect that the government deliberately withheld that information from the public.

I do not know whether or not there was a deliberate intent by the government to withhold information. Joseph Smadel, Bernice Eddy’s immediate superior at the NIH and himself an eminent scientist, dismissed Eddy’s finding of a tumor-inducing virus in early lots of the Salk vaccine, after he reviewed her data. Later, after Eddy reported her findings at an October 1960 cancer conference in New York, Smadel forbade her from speaking about the matter again in public without first clearing her remarks with him. However, Smadel was indeed concerned when similar findings were reported to him later by Hilleman. Was sexism a factor in the dismissal of Eddy’s results? Perhaps, but it also was alleged (in fact by Hilleman) that Eddy’s experiments were poorly controlled.

At any rate, Eddy had, in fact, presented her findings at an open scientific conference. What’s more, at around the same time, Hilleman too presented his findings in public, at a conference in Copenhagen. So, their discovery, while not purposely publicized, was not kept secret either.

Next, consider that the second report in the New York Times, which mentioned the cancer connection for the first time, was buried on page 27 of the newspaper. This fact points more to the inability of the press to appreciate and cover a complex scientific issue, than to an attempt by the government to suppress the story.

In any event, the government did not alert the public to the possible danger that might be lurking in the polio vaccines. Is it possible that the NIH did not appreciate that threat? This seemingly implausible explanation is consistent with the fact that the NIH did not begin to screen all new lots of the polio vaccines for SV40 until 1963.

Another perhaps more likely possibility is that the government simply believed that alerting the public might have irrevocably broken its confidence in the vaccines, ultimately causing far more disease than might have been caused by the vaccines themselves. Even so, an impending public health debacle, of unprecedented severity, could not yet have been ruled out.

We return to our main story after a bit of background on SV40.

SV40 is a member of the polyomavirus family of small, double-stranded DNA viruses. It is one of several polyomaviruses that can transform normal cells into tumor cells in cell culture, as well as induce tumors in laboratory animals. For that reason, and because SV40 is a relatively simple virus (its genome contains only about 5,200 bases pairs), it was intensively studied for what it might reveal about tumor genesis. Moreover, it also came to serve as an important model to investigate fundamental issues in eukaryotic molecular biology (3).

The polyomaviruses are widespread in their natural hosts, in which they give rise to lifelong, usually benign, persistent infections. The Asian rhesus macaque is the natural host for SV40. These facts explain why SV40 was not evident in the rhesus monkey kidney cell cultures that were used to propagate the early poliovirus vaccine lots. SV40 does not cause sufficient cytopathology in those rhesus macaque cell cultures to reveal its presence in them. However, culture fluids from those rhesus cell cultures caused extensive cytopathology when added to African green monkey kidney cell cultures. Indeed, that is how SV40 was discovered.

[Aside 2: The Human Polyomaviruses The JC polyomavirus (JCPyV) and the BK polyomavirus (BKPyV), each discovered in 1971, are the best known polyomaviruses that naturally infect humans. JCPyV and BKPyV are each ubiquitous in their human host, in which they typically give rise to lifelong, benign, persistent infections. Yet, JCPyV can give rise to a rare but fatal demyelinating disease, progressive multifocal encephalopathy (PML), in immunologically compromised individuals. And BKPyV can cause kidney disease, also in immunologically compromised persons.

More recently, several additional human polyomaviruses have been discovered, by means of modern DNA amplification procedures. One of these viruses, the ubiquitous Merkel cell polyomavirus (MCPyV), is associated with a rare, aggressive human malignancy, Merkel cell carcinoma, and is the best candidate for an oncogenic human polyomavirus.]

We now resume our main story, with the following key points.

Despite the fact that the unintended exposure of millions of individuals to SV40 via the contaminated polio vaccines in the 1950s posed a potential public health crisis of immense proportions, it still is not clear whether SV40 is an agent of human disease. Moreover, it is not known whether SV40 is circulating in the human population. How can this be?

Early investigations into this matter in the 1960s were compromised by the fact that it was not apparent which individuals had actually received SV40-contaminated vaccines and which did not. That was so in part because the serological reagents and procedures of the day were not sensitive or accurate enough to generate unambiguous results. Additionally, population sample sizes were often too small to generate statistically significant results. [This was especially so in the case of the rare childhood tumors in which SV40 had been implicated.] And, since cancer is a disease that may take decades to emerge, it was possible that more time needed to pass before the virus might unequivocally reveal itself as a cause of human cancer. At any rate, since the substantial experimental data then available could neither establish nor absolve SV40 as a cause of cancer in humans, the NIH conceded that more research and better methods for detecting the virus would be needed to settle the issue.

The more recent development of extremely sensitive polymerase chain reaction (PCR)-based procedures, which can detect minute levels of specific DNA sequences, led to renewed interest in whether SV40 might be present in humans, and whether it might be an agent of human disease. Using PCR technology, several different research groups detected SV40 DNA in four types of human cancers; mesotheliomas, osteosarcomas, non-Hodgkin’s lymphomas, and childhood brain tumors. These findings were alarming because the four tumor types, in which SV40 was detected in humans, are the same tumors that SV40 induces experimentally in hamsters (i.e., mesothelioma, bone, lymphoma, and brain).

PCR procedures also detected SV40 DNA in individuals who never were inoculated with an SV40-contaminated vaccine. This too was disturbing because it raised the specter that SV40 might be circulating in the human population, spreading by horizontal transmission from one individual to another.

Yet the issue of SV40 in humans remains controversial because other studies, from other research groups, using similar PCR procedures, could not detect SV40 DNA in human tissues. In addition, newer, more sensitive and accurate serologic procedures could not demonstrate to everyone’s satisfaction that SV40 circulates in humans.

How might we explain how capable scientists, using powerful and proven techniques, can obtain such disparate experimental results? Ironically, the sensitivity of PCR itself may be a problem, since it increases the likelihood of false-positive results, which may occur from the slightest sample contamination. Thus, it is important to have suitable positive and negative control samples that might be processed side-by-side with test samples; a sometimes difficult criterion to fulfill. [Bearing the above in mind, consider the following example, in which a human mesothelioma sample was micro-dissected to separate normal tissue from the actual tumor. SV40 sequences were detected in the tumor, but not in the adjacent normal tissue, which served as an internal control.]

Another potential source of error stems from the widespread prevalence of human polyomaviruses (e.g., JCPyV, BKPyV, and MCPyV) in the human population, leaving open the possibility that these viruses, rather than SV40, are detected by the PCR-based procedures. But, with that possibility in mind, several researchers took the extra step of confirming the presence of SV40 sequences by direct sequencing of the PCR-amplified DNA. The ubiquitous human polyomaviruses are also a concern when carrying out serological procedures, since immune cross-reactivity between SV40 and these viruses remains a potential source of error.

Another problem is theoretical rather than technical. An underlying premise behind these studies is that the continued presence and expression of polyomaviral tumor genes are necessary for a tumor cell to express its tumor cell characteristics. Indeed, this was shown to be the case fifty years ago for cells transformed in culture by polyomaviruses. Thus, the absence of SV40 DNA, or SV40 tumor antigens, in a tumor is taken as evidence against SV40 as the cause of the tumor. However, there is some experimental evidence that the paradigm itself may not always be true. Cancers result from a complex multistage course of events and, in some instances, the virus may play a necessary role only at a particular point in the overall process. Thus, the absence of SV40 DNA, or SV40 antigens, in a tumor may not be definitive proof against viral involvement in the tumor process.

The above points help us to appreciate why there is no consensus regarding a role for SV40 in human cancer, and indeed whether SV40 might be circulating in humans. Yet we remain troubled by the fact that SV40 can transform a variety of cells in culture and can induce tumors in laboratory animals. And there are additional experimental findings that while contentious, cannot be easily ignored. For instance, the types of human cancers, in which SV40 DNA was detected by some researchers, are the same types of tumors that SV40 induces in laboratory animals. Also, infectious SV40 was isolated from a brain cancer of a 4-year-old child. [LT/pRb and LT/p53 complexes were identified in human brain tumors, consistent with current understanding of how SV40 induces neoplasia (3).]

Yet, notwithstanding the force of the above arguments, impressive evidence has been presented against a role for SV40 in human cancer. And, if SV40 indeed were responsible for human cancer on a large scale, then it is rather certain that there would be little if any uncertainty in that regard.

1. Jonas Salk and Albert Sabin: One of the Great Rivalries of Medical Science, posted on the blog, March 27, 2014

I recently watched the fact-based 2009 movie Endgame, which depicts the final days of apartheid in South Africa. The movie focused on how a young Thabo Mbeki facilitated the late 1980s secret talks between Afrikaner leaders and the about-to-be freed Nelson Mandela’s African National Congress. The purpose of these talks was to arrange negotiations between de Klerk’s apartheid government and the ANC, in order to facilitate the stable transition of South Africa from apartheid.

Thabo Mbeki with Nelson Mandela

Mbeki is portrayed in the film as a wise, compassionate, and respected political activist. Moreover, all commentary on the movie that I’ve read is consistent with it having accurately depicted people and events. Here then is Mbeki in a different guise, when dealing with the South African AIDS epidemic, after succeeding Mandela as the nation’s president. As usual, we begin with some background.

The South African AIDS outbreak may well have been the most devastating of all the world’s AIDS epidemics. Consider the following statistics. By 2007, more than one in five South African adults (approximately 5 million people) were living with HIV, and the disease claimed nearly 1,000 lives daily. Shockingly, 71% of all deaths among individuals between 15 and 49 years of age were due to AIDS. Moreover, a 15-year old South African had a 50:50 chance of dying of AIDS before his or her 30th birthday.

Yet despite these statistics, South Africa escaped the initial HIV/AIDS epidemic of the 1980’s. Why then did HIV/AIDS get so out of hand in South Africa in the early 21st century? One reason is that it was a time when the government was preoccupied with the nation’s poverty and its transition from apartheid. Moreover, and importantly, any rational efforts to deal with the South African HIV/AIDS epidemic were severely compromised when, in 2001, President Mbeki embraced the “denialist” view that AIDS is not caused by HIV. [See the addendum on the AIDS denialists at the end of this posting.]

Mbeki’s “denialist” position on AIDS dates back to 2001, when his minister of health convened a 36-member international panel of supposed AIDS experts to advise Mbeki on how best to confront the nation’s AIDS crisis. Regrettably, the14 known HIV/AIDS denialists, who were invited by the health minister to join Mbeki’s AIDS Advisory Panel, convinced Mbeki to adopt their denialist point of view.

Afterwards, Mbeki and his administration were repeatedly accused of improperly dealing with their nation’s AIDS epidemic. Noting that AIDS patients succumb to the opportunistic infections that appear following the HIV-caused breakdown of their immune systems, South African AIDS patients, who used the public health system, were able to get treatment for their opportunistic infections. However, the Mbeki government prevented those patients from receiving antiretroviral therapy, which might have prevented the breakdown of their immune systems in the first place.

Governmental restrictions against the use of antiretroviral drugs remained in place until August 2003, when the South African Cabinet overruled Mbeki, and declared as Cabinet policy that HIV is the cause of AIDS. Moreover, the cabinet promised to formulate a national treatment plan that would include antiretroviral therapy. Yet despite the efforts of the cabinet to make antiretroviral drugs the mainstay of the country’s treatment plan, the health minister continued to promote nutritional approaches to treating AIDS, while also proclaiming the toxicity of the antiretroviral drugs.

The continued unwillingness of the Mbeki administration, to acknowledge that HIV is the cause of AIDS, prevented the cabinet’s more enlightened AIDS policy from being fully realized. By the end of 2007, only about 28% of South Africa’s AIDS patients were able to obtain antiretroviral therapy. And, since many HIV-infected pregnant women were not getting the antiretroviral drugs that might have prevented transmission of the virus to their babies, HIV remained tragically common among South African children. In this regard, in 2007, UNAIDS reported that there were about 280,000 children under 15 years of age who were living with HIV in South Africa. Moreover, AIDS-related adult deaths resulted in 1.4 million South African children becoming orphans in 2007; a rise from the 780,000 new orphans reported in 2003.

A 2008 report from the Harvard School of Public Health estimated that Mbeki’s denialist stance caused an estimated 330,000 or more preventable AIDS-related deaths in South Africa. That estimate was based on comparing the effect of antiretroviral therapy in neighboring Botswana and Namibia, to the state of affairs in South Africa, during the period from 2000 to 2005.

Mbeki resigned the presidency in September 2008, after losing the support of his party. His resignation was not related to his AIDS denialism. Rather, it involved his alleged interfering with the country’s National Prosecuting Authority in a political matter.

Following Mbeki’s resignation, his successor, Kgalema Motlanthe, fired health minister Manto Tshabalala-Msimang, who, like Mbeki, was an AIDS denialist. South Africa then initiated the largest antiretroviral therapy roll-out program in the world, resulting in a 5-year jump in life expectancy to a current 36 years-of age. Moreover, in 2011, there were 100,000 fewer AIDS-related deaths in South Africa than occurred in 2001, and over 300,000 fewer than occurred in 2006. Still, about 5.6 million South Africans were estimated to be infected with HIV in 2011; the highest number of infected people in any country! Almost one-in-three women aged 25-29, and over a quarter of men aged 30-34, were HIV-positive

Considering the prevalence of HIV in South Africa, it may surprise some readers that there is widespread prejudice in the country against those living with AIDS. With that in mind, we note Nelson Mandela’s action when his son died of AIDS in 2005. Mandela deliberately revealed the cause of his son’s death to the public. His purpose was to countermand the stigma associated with being infected with HIV and, also, as a “political statement designed to… force the President [Mbeki] out of his denial.” Earlier, in 2002, South African politicians, who were loyal to Mbeki, attacked Mandela for questioning the government’s AIDS policy.

Bearing in mind that Mbeki was a respected and intelligent leader, how might we explain his rejection of the orthodox view, based on indisputable evidence, that HIV is the cause of AIDS? Could he simply have been taken in by the AIDS denialists? Or, might there be more? A likely possibility is as follows. Mbeki was known to harbor strong anti-colonialist and anti-West sentiments, born of having come of age during South Africa’s apartheid era. Perhaps his earlier experiences caused him to see his country’s AIDS crisis as a means by which the West sought to exploit his nation’s people.

In the same vein, Mbeki was acutely conscious of South Africa’s poverty and, consequently, was likely affected by the huge expense of antiretroviral drugs. In that regard, his health minister advised him that the cost of treating the 5 million or so HIV-infected South Africans with AZT would exceed the annual health department budget by a factor of ten. And, given Mbeki’s anti-West sentiments, he was particularly sensitive to the fact that antiretroviral drugs were made and sold by powerful Western pharmaceutical companies. Additionally, Mbeki may have been skeptical of the efficacy of those drugs; an attitude reflecting the fact that the health policies of the colonial and apartheid governments in South Africa indeed were often self-serving and manipulative.

Mbeki indeed accepted that AIDS results from the collapse of the immune system. Nevertheless, he believed (or at least claimed) that the cause of that collapse was poor nutrition and poor health resulting from poverty, rather than from a virus. Thus, he argued that he needed to attend to poverty in general, rather than to AIDS in particular.

Treating HIV/AIDS has indeed been expensive for South Africa. Additionally, the more than $1 billion that the nation now spends annually on its HIV/AIDS program has been largely financed from its own domestic resources. Yet, South Africa is still desperately trying to emerge from the poverty of its former oppression. So, if you were in Mbeki’s position as the President in 2001, would you have chosen to support AIDS therapy, which might have exceeded the limits of your financial resources for the foreseeable future or, instead, would you invest in water systems, housing, schools, and hospitals? And what of other crucial social and medical problems that still abound in South Africa, such as malaria, tuberculosis, and violence against women? You cannot have it all. And even if Mbeki and his government had believed in and fully supported antiretroviral therapy, impediments to its effectiveness would still have remained. These comment are not meant to defend Mbeki, but, instead, to point up the complexity of the AIDS problem in a poverty-stricken nation emerging from apartheid.

Addendum: The AIDS denialsts: While some AIDS denialists had legitimate scientific credentials, none, except Peter Duesberg, was an expert retrovirologist. Indeed, before Duesberg emerged as an AIDS denialist, he made important contributions to the retrovirus field, including his finding that Rous sarcoma virus contains an oncogene (1). For this contribution, and others, Duesberg was elected to the prestigious U.S. National Academy of Sciences. However, once Duesberg broke ranks with other scientists over AIDS, he became a scientific outcast, no longer receiving invitations to scientific conferences and no longer able to obtain research grants.

Duesberg advocated the belief that AIDS results from drug abuse, parasitic infections, and malnutrition, rather than from a retrovirus, and that HIV itself is just another opportunistic infection. He stated that if he discovered that he was HIV antibody-positive, he would take that as an encouraging sign that his immune system was working.

Yet Duesberg was never able to offer any plausible evidence that might substantiate his dissident views. Moreover, he repeatedly ignored the numerous rebuttals of his claims that appeared in the scientific literature. [Kary Mullis, who was awarded the Nobel Prize for inventing the polymerase chain reaction (PCR), is another AIDS denialist with legitimate, indeed prestigious scientific credentials. But, unlike Duesberg, Mullis has no expertise that might be relevant to HIV/AIDS.]

In an interesting aside, in 2009, Duesberg published a paper in Medical Hypotheses, which defended Mbeki and disputed the 2008 study which reported that hundreds of thousands of lives were lost in South Africa because antiretroviral drugs were not available to AIDS patients there. Prominent AIDS researchers, including Nobel laureate Francoise Barre-Sinoussi (2) then complained to the journal’s publisher, Elsevier, asking that the paper (which had not yet been printed) be withdrawn.

Since Medical Hypotheses was the only Elsevier journal to have a policy against peer review, Elsevier then asked the editors of another of its journals, The Lancet, to oversee review of Duesberg’s paper. After The Lancet reviewers found that the paper contained numerous errors and misinterpretations, Elsevier permanently withdrew it. What’s more, Elsevier then forced Medical Hypotheses to introduce peer review. [On the one hand, the editorial policy of Medical Hypotheses, to “consider radical, speculative and non-mainstream scientific ideas,” may well have provided a means for airing new, potentially important premises. On the other hand, publication of Duesberg’s denialist notions, in what is nominally a scientific journal, would have given those notions an air of credibility, potentially impairing worldwide efforts against AIDS.]

Far from conceding his position, Duesberg claimed that Elsevier’s measures are the latest example of “censorship” imposed by the “AIDS establishment.” He then published a revised version of the paper in the Italian Journal of Anatomy and Embryology, causing further controversy.

So, taking into account Duesberg’s very real expertise as a retrovirologist, can he possibly have been right about HIV and AIDS, and is his alternative view helpful in the fight against the disease? It would make for a fascinating story if the answers were yes, or just even maybe. But, the evidence that HIV causes AIDS is, without exaggeration, overwhelming. Consider just the following. Data from matched groups of homosexual men and hemophiliacs show that only those who are infected with HIV ever develop AIDS. Moreover, in every known instance where an AIDS patient was examined for HIV infection, there was evidence for the presence of the virus. These data have been available for years, and Duesberg should have been well aware of them. What is more, there has been the enormous success of antiretroviral therapy in changing AIDS from a nearly invariably fatal disease, into a manageable one, for many HIV-infected individuals.

It is not known how many people might have been infected with HIV, or might not have benefited from effective antiretroviral therapy, because they heeded the arguments of the AIDS denialists. These individuals continue to tout their notions, to the detriment of the millions of HIV-infected individuals who listen to them.

1. The relevance of retroviral oncogenes is discussed in: The Politics of Science: Vignettes Featuring Nobel Laureate Harold Varmus during his Tenure as Director of the NIH, posted on the blog, June 2, 2014.

Blogs I Follow

Welcome!

I am now a retired professor emeritus of Microbiology at the University of Massachusetts. Teaching virology has been a most rewarding aspect of my career. I especially enjoyed enlivening my lectures with a variety of relevant anecdotes.

Virology Textbook

Based on my experiences teaching virology for more than 35 years, I wrote Virology: Molecular Biology and Pathogenesis (ASM Press; 2010). For info on adopting or buying this textbook, please visit the publisher site: http://www.asmscience.org/content/book/10.1128/9781555814533